1. Field of the Invention
The present invention relates to a laser radar and a beam irradiation apparatus therefor, particularly to a vehicle-mounted laser radar and a beam irradiation apparatus therefor.
2. Description of the Related Art
Recently, the laser radar is mounted on an automobile in order to enhance safety during running. The beam irradiation apparatus which irradiates ahead with a laser beam is mounted on the laser radar, and an obstacle is detected based on existence of the reflected light when the beam irradiation apparatus irradiates ahead with the laser beam. A distance to the obstacle is measured based on a temporal difference between a laser beam emission time and a reflected light acceptance time.
At this point, means for scanning a predetermined target region with the laser beam is provided in the beam irradiation apparatus. For example, a lens driving type beam scan mechanism can be used as the means for scanning the target region. In a configuration of the lens driving type beam scan mechanism, the target region is scanned in a two-dimensional direction with the laser beam by two-dimensionally driving a beam scanning lens supported by a wire, which allows the beam scan to be realized with high reliability.
However, in the configuration, unfortunately the lens and a driving mechanism thereof are enlarged and a large driving power is necessary to drive the lens.
Alternatively, a gimbal type actuator can be used as other means for scanning the target region with the laser beam. In the gimbal method, a beam scanning mirror is rotated about two rotating axes orthogonal to each other, thereby scanning the target region in the two-dimensional direction with the laser beam. According to the gimbal type actuator, compared with the lens driving method, downsizing of the actuator can be achieved and the driving power necessary to drive the mirror can be decreased.
Generally, in the gimbal type actuator, while a rotation position of the mirror is fixed in a vertical direction, the mirror is rotated in a horizontal direction to scan the target region with the laser beam. When one-line horizontal scan is ended, the mirror is rotated by a predetermined angle in the vertical direction, and the mirror is rotated in the horizontal direction to perform the horizontal scan for the next line. The whole of target region is scanned by repeating the operation.
However, when the mirror is rotated in the horizontal direction while the rotation position of the mirror is fixed in the vertical direction, the region (hereinafter referred to as “scan region”) scanned with the laser beam is formed into not a rectangular shape but a shape having an outline deformed in a longitudinal direction or a crosswise direction. On the other hand, in the laser radar, generally a rectangular (horizontally long rectangle) region is set as the scan region. Therefore, in the case where the mirror is driven in the above-described manner, because the scan region does not become the desired rectangular shape, it is a possibility that obstacle detection and the distance measurement cannot be properly performed.